An internal-combustion engine vehicle using fossil fuel such as gasoline or diesel creates air pollution due to exhaust gas, global warming due to carbon dioxide, respiratory diseases due to generation of ozone, and the like.
An eco-friendly vehicle has been developed, such as an electric vehicle (EV) driven by electric power, i.e., by driving an electric motor, a hybrid electric vehicle (HEV) driven by an internal-combustion engine and an electric motor, and a fuel cell electric vehicle (FCEV) driven by driving an electric motor using electric power generated from a fuel cell.
In such an eco-friendly vehicle, braking or inertia energy during braking of the vehicle or during coasting by inertia can be collected as electric power is generated by an electric motor and then charged in a battery (motor charge) in a regenerative mode.
As such, energy is collected using the electric motor in the braking or coasting of the eco-friendly vehicle, and the battery is charged with the collected energy. Thus, the energy is efficiently used, and thus, the fuel efficiency of the vehicle can be improved.
More specifically, the eco-friendly vehicle has a regenerative braking system converting kinetic energy of into electrical energy during braking of the vehicle and storing the converted energy in the battery. The regenerative braking system enables the stored energy to be used in driving the electric motor, thereby improving the fuel efficiency of the vehicle.
A vehicle controller (hybrid control unit (HCU) or vehicle control unit (VCU)) for controlling all operations of vehicle is provided in the vehicle. In addition, the eco-friendly vehicle has a variety of controllers for controlling various devices of the vehicle.
For example, the eco-friendly vehicle has a brake controller for controlling the operation of a friction brake (friction braking device), a motor controller (motor control unit (MCU)) for controlling the operation of a motor, a transmission controller (transmission control unit (TCU)) for controlling the operation of a transmission, a battery controller (battery management system (BMS)) for collecting information on battery state of charge (SOC). The eco-friendly vehicle then uses the collected information in battery charging/discharging control or provides the collected information to the other controllers, and the like.
The vehicle controller and each controller perform cooperation control while communicating information with each other through controller area network (CAN) communication. An upper-level controller among the controllers transmits control commands to the lower-level controllers while receiving and collecting various information transmitted from the lower-level controllers.
Main functions of controllers related to regenerative braking in the eco-friendly vehicle will be briefly described with reference to FIG. 1. When a driver braking request is generated, a brake controller 10 calculates a regenerative braking permissible amount and transmits the calculated regenerative braking permissible amount to a vehicle controller 20.
In consideration of a regenerative prohibition situation, etc., the vehicle controller 20 calculates a motor request torque, i.e., a motor torque command (regenerative braking command) according to the regenerative braking permissible amount, and transmits the calculated motor torque command (regenerative braking command) to a motor controller 30. The vehicle controller 20 estimates a regenerative braking execution amount according to each state of a motor, a transmission.
The motor controller 30 controls the motor through an inverter according to the motor torque command (regenerative braking command) received from the vehicle controller 20, and transmits information on a motor output torque (actual motor torque) so that the vehicle controller 20 can estimate the regenerative braking execution amount.
The brake controller 10 calculates a total braking amount according a driver braking request (manipulation of a brake pedal), and distributes braking power according to the total braking amount, i.e., distribution of regenerative braking power and friction braking power with reference to the regenerative braking execution amount transmitted from the vehicle controller 20.
That is, the brake controller 10 determines a friction braking amount obtained by subtracting the regenerative braking execution amount from the total braking amount according to the driver braking request, and then controls the operation of a friction brake (friction braking device) to generate braking power corresponding to the friction braking amount.
A transmission controller 40 controls a vehicle transmission such as an automatic transmission (AT) or a dual clutch transmission (DCT), and transmits information on a transmission state (transmission gear ratio), etc. to the vehicle controller 20 so that the vehicle controller 20 can estimate the regenerative braking execution amount.
For a decelerator-type electric vehicle, there is no transmission controller.
In the conventional eco-friendly vehicle, the brake controller 10 calculates a regenerative braking permissible amount, the vehicle controller 20 calculates a regenerative braking command (motor torque command) according to the regenerative braking permissible amount, and the motor controller 30 performs motor control according to the motor torque command.
If the motor controller 30 calculates the actual motor torque (actual output torque) and transmits the calculated actual motor torque (actual output torque) to the vehicle controller 20, the vehicle controller 20 calculates the regenerative braking execution amount using information on each state of the motor and the transmission, i.e., information on the actual motor torque and the transmission gear ratio, and then transmits the calculated regenerative braking execution amount to the brake controller 10. The brake controller 10 distributes the braking power using the regenerative braking execution amount and then controls the friction brake to generate the distributed friction braking power.
Here, the regenerative braking execution amount in the vehicle controller 20 is calculated from ‘regenerative braking execution amount=motor actual torque×transmission gear ratio (or speed reduction ratio)−coasting torque.’ In the case of an HEV, when an engine is connected to a motor through coupling of an engine clutch, a fuel cut state of the engine is reflected to the actual motor torque.
However, communication delay exists between the controllers, and the vehicle controller 20 calculates a regenerative braking execution amount based on an actual torque of the motor controller 30. Hence, a considerably long period of time is required until the brake controller 10 calculates a total braking amount and then distributes the braking power by receiving the regenerative braking execution amount from the vehicle controller 20. Therefore, accurate distribution of braking power and actual reduction in speed of the vehicle are not performed in an urgent moment.
For example, if it is assumed that, when the vehicle controller 20 calculates the regenerative braking execution amount based on the actual motor torque transmitted from the motor controller 30, the CAN communication period between the controllers is 10 ms which is a duration in which the regenerative braking execution amount is fed back to the brake controller 10 after generation of a driver braking request is as shown in FIG. 2.
That is, there exists a delay of 50 ms or less which is generated from ‘driver braking input→calculation of regenerative braking permissible amount in brake controller (10 ms)→calculation of regenerative braking command (motor torque command) in vehicle controller (10 ms)→control of motor and calculation of motor actual torque in motor controller (10 ms)→calculation of regenerative braking execution amount in vehicle controller (10 ms)→distribution of braking power in brake controller (10 ms)’.
The communication delay causes considerable inaccuracy in view of braking distribution, and the braking performance of the vehicle is deteriorated.